docs: add AI documentation suite and comprehensive code review report
- Generate full AI-consumable docs (docs/ai/): system overview, architecture,
module cheatsheet, API contracts, data model, build guide, decision log,
glossary, and open questions (deep tier coverage)
- Add PROBLEM_REPORT.md: categorized bug/risk summary
- Add DETAILED_CODE_REVIEW.md: full line-by-line review of all 15 backend
files, documenting 4 fatal issues, 5 critical deployment bugs, 4 security
vulnerabilities, and 6 architecture defects with prioritized fix plan
22787b3e0a